Painting designs onto object
Create the designs in photoshop or whatever image editing program you use. Export them with alpha.

In your material node, add a new image texture and mix it with your regular wood texture. Go to texture paint mode and load your art from photoshop as a texture for the brush. Set the brush mapping to Stencil and orient your view appropriately.

Here's a tutorial on how stenciling works in Blender: https://www.youtube.com/watch?v=VnfTSXWF21A